1. Field of the Invention
This invention relates to a safety interlock system for an apparatus which generates either electron radiation or X-ray radiation. Such an apparatus is used e.g. for the medical treatment of patients.
2. Description of the Prior Art
It is known in the art of radiation systems to switch-off an unwanted radiation beam by utilizing an ionization chamber to which the radiation is applied, as soon as a previously determined dosage of radiation has been reached. U.S. Pat. No. 4,347,547 describes such a radiation system in which a linear accelerator emits electron pulses which are directed to a target for the generation of X-ray pulses. The ionization chamber is exposed to the X-ray pulses for measuring their intensity distribution. A discriminator is connected to the ionization chamber for detecting intensity inhomogeneities in the X-ray pulses. If the energy of the X-ray radiation is not between a predetermined maximum value and a predetermined minimum value, a switch is operated by the discriminator and switches off the accelerator by inhibiting the power supply of the accelerator. Simultaneously, there may also be stopped the high voltage supply to the accelerator, an RF voltage of a high frequency (HF) source and/or the injection of electrons into a waveguide of the accelerator.
U.S. Pat. No. 4,342,060 discloses another safety interlock system for a linear accelerator. A measuring device determines the level of the particle beam pulses emitted by the accelerator through a target which is exposed to the particle beam pulses. A discriminator determines whether the level of the particle pulses is higher than a predetermined value. If this is the case then a switch is operated which switches off the power supply of the accelerator, the RF signals of a HF power source and/or the emission of electrons of an electron gun of the accelerator.
From U.S. Pat. No. 4,115,830 a monitoring system for a high voltage supply of an ionization chamber is known. This system is preferably used for monitoring a particle accelerator in order to regulate the radiation intensity or the radiation output via the ionization current of the ionization chamber subjected to the radiation.
There have been known systems which are able to generate either electron radiation or X-ray radiation. In the case of generating electron radiation, a scattering foil is arranged at an exit window of the accelerator in the trajectory of the emitted electron beam. In case of generating X-ray radiation a target is arranged at the exit window of the accelerator in the trajectory of the electron beam and the particles emitted by the accelerator have high intensity so that they can generate enough bremsstrahlung for the generation of the X-rays. Such systems have been used e.g. for the medical treatment of patients with electron radiation or with X-ray radiation.
If a failure occurs during the operation of such a system and the particles having high intensity, like during the generation of X-ray radiation, are emitted by the accelerator and the scattering foil is positioned in the trajectory of the electron beam although the target should be in this position, the patient is exposed to a very high electron radiation and this could be very harmful to a patient.
If the radiation is measured by the ionization chamber according to the above noted prior art technique, there is still a certain risk that the patient receives too much radiation, because the accelerator is not switched-off, until after the radiation has left the accelerator and is measured and determined to be too great while already on its path to the patient.